The invention discloses a method for detecting the thickness reduction of the heated surface tube of a boiler. The method comprises the following steps: 1, obtaining the frequency dispersion curve ofLamb waves under the standard wall thickness d1 of the heated surface tube in the boiler to be detected; 2, obtaining an optimal detection frequency f1 according to a group velocity frequency dispersion curve in the frequency dispersion curve; 3, obtaining the phase velocity cp of the Lamb waves in a phase velocity frequency dispersion curve, corresponding to the optimal detection frequency f1 instep 2, and determining the incidence angle alpha of an acoustic beam according to the Snell's law; 4, allowing a variable angle probe to connect with an ultrasonic flaw detector and detect at the optimal detection frequency f1 according to the incidence angle alpha of the acoustic beam in step 3, and acquiring echo signals in an S0 mode and an A0 mode; and 5, obtaining the transit time differencebetween S0 mode echo and A0 mode end face echo waves according to the echo signals, and calculating the thickness reduction of the heated surface tube according to the transit time difference. The method has the advantages of convenience and fastness in completing of detection, and increase of the reliability of a detection result.